Recently, recording density of magnet disk units is dramatically increasing. This is largely attributable to a highly-sensitive MR head (Magneto-Resistive effect head). Simultaneously, this is also largely attributable to practical use of PR4ML (Partial Response class 4 Maximum-Likelihood) method which can perform regeneration with low S/N ratio as signal processing method, rather r than conventional peak detection method. The PR4ML method removes waveform intervention by utilizing partial response, reduces noises by lowering a frequency band, and performs convolution according toU(T)=(1−D)·(1+D)n 
D: delay operator indicating one (1) bit delay,
and then, finds a signal with maximum-likelihood from regeneration signals containing disturbances such as noises by a maximum-likelihood detecting circuit with Viterbi algorithm. At this point, the convolution of (1+D) has the transfer constant shown in FIG. 1, and is improved to increase the order n to 2 or 3, in order to reduce noises of high-frequency band. As a result, though the maximum-likelihood detecting circuit becomes more complicated, error rate is improved. The order n is limited up to 3, and in case that it was increased to more than 3, improvement of the error rate was little. In the noises regenerated in magnetic recording, the large power noises are contained in a low-frequency band, rather than a high-frequency band. Most of the noises in the high-frequency band are distributed substantially uniformly throughout the bandwidth of noises such as pre-amplifier noises and MR head noises. Contrary, a peak of power of medium noises is present relatively near the low-frequency band, partially depending on materials of recording medium. Recently, since an area of one (1) bit is closing in on that of particles of magnetic materials, the medium noises are increasing, therefore some recording medium have the marked peak of power in low-frequency band, as showing in FIG. 2. Also, side-crosstalk from adjacent tracks becomes larger in lower frequency band, because of the nature thereof. Conventionally, it is believed that the convolution of (1+D) is enough for the noises in low-frequency band. In this point, it is believed that the convolution of (1+D) has transfer characteristic showing in FIG. 3, and reduces the noises in low-frequency band. However, since this convolution in magnetic recording is actually performed in a recording system before generation of the medium noises, and an equalizing target thereof is characteristic showing in FIG. 4, therefore the convolution of (1+D) does not act on the noises in low-frequency band at all, and medium noises is not affected by the transfer characteristic with the convolution of (1+D).